JSON numbers are not equivalent to JavaScript/ECMAScript numbers, even if they are nominally related; the key differences are that in JSON, numeric literals:

(a) can have any non-zero number of digits, effectively making JSON numbers both unbounded and arbitrarily precise (though actual infinities cannot be represented); and

(b) cannot represent values that are not composed of digits, like NaN.

For that reason, most standard (fixed size/binary) numeric types like double are a poor choice to contain numeric values specified in JSON; in particular, the mismatch means that conversion can be lossy in both directions.

Hope that helps!

Alvaro

On Tue, Apr 24, 2012 at 5:19 PM, Jeremy Shaw <jeremy@n-heptane.com> wrote:

Hello,

I could be wrong, but I think the only real numeric type in javascript
is 'Number' which is a floating point number? Which is why Aeson and
others insist on converting everything to a Double or other Rational
number?

- jeremy

On Tue, Apr 24, 2012 at 3:46 PM, Jeff Shaw <shawjef3@msu.edu> wrote:
> Hello,
> Up until now I've been using Aeson, but I've found that its number type
> isn't going to work for me. I need to use decimal numbers while avoiding
> conversions from and to Double, which Aeson doesn't allow. There are quite a
> few more JSON libraries for Haskell, which all appear to use Rational for
> numbers, so I'm wondering if anyone can recommend one.
>
> Thanks,
> Jeff
>
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